Various forms of line interface circuit, and various desirable features in line interface circuits, are known. In particular, it is known for example from Rosenbaum U.S. Pat. No. 4,484,032 issued Nov. 20, 1984 and entitled "Active Impedance Transformer Assisted Line Feed Circuit" to provide a line interface circuit in which two amplifiers have their outputs coupled to the tip and ring wires of a two-wire telephone line via feed resistors and primary windings of a very small transformer. In this arrangement, a secondary winding of the transformer and a resistive network coupled to the feed resistors provide for sensing of a.c. and d.c. conditions on the line.
It is also known from Bolus et at. U.S. Pat. No. 4,431,868 issued Feb. 14, 1984 and entitled "Solid State Telephone Line Interface Circuit With Ringing Capability" to provide a line interface circuit in which a low-level ringing reference signal can be supplied to the input of high-voltage tip and ring wire amplifiers, to be amplified thereby to produce a desired high voltage ringing signal on the line. The supply voltage rails of the amplifiers are controlled to follow, with an offset, the instantaneous values of the ringing signal waveform in order to reduce power dissipation. D.c. levels at the amplifier outputs are determined by d.c. potentials at the amplifier inputs and the d.c. gain of the amplifiers. This arrangement has the disadvantage that the amplifiers must operate with a substantial gain to produce the large signal amplitudes at their outputs, and consequently have a relatively narrow bandwidth.
With evolution of telephone systems, it is desirable to provide improved line interface circuits which in particular have a bandwidth which is sufficiently great to accommodate ISDN (integrated services digital network) services, for example a signal bandwidth of the order of 200 kHz. At the same time, it is desirable to provide improvements in line interface circuits with respect to such features as their size, cost, versatility, and operation especially in relation to fault conditions, common mode signal rejection, and power consumption and dissipation.
Considered generally, there is a need for a line interface circuit which can be used to operate in conjunction with any arbitrary telephone communications line to provide arbitrary voice and data communications services as may be desired at any particular time and which the line is capable of carrying, which services can be readily changed under software control from a telephone central office processor without requiring any hardware changes of the line interface circuit.
While the line interface circuit described in U.S. Pat. No. 4,484,032 provides considerable advantages over other line interface circuits, in particular in allowing precise longitudinal balance and the use of a very small transformer facilitating improved noise performance and bandwidth which is independent of the resistance values used in the resistive network, which accordingly can be high to minimize power consumption and dissipation, nevertheless some disadvantages remain, in particular relating to noise susceptibility, a residual non-uniformity in the frequency response, and an inability to monitor common mode current separately from differential or loop current, as is desirable in a highly versatile line interface circuit. These disadvantages are discussed further in the detailed description in this specification.
In order to provide cancellation of common mode signals, and to provide a substantially constant threshold for ground fault current limiting, it is known from Rosch et at. U.S. Pat. No. 4,764,956 issued Aug. 16, 1988 and entitled "Active Impedance Line Feed Circuit" to connect a potential divider between the tip and ring wire voltage taps of the resistive network, in a transformerless line interface circuit, and to connect a d.c. amplifier to the tapping point of the potential divider. The output of the d.c. amplifier is coupled to the voltage taps via oppositely-poled parallel diodes and resistors, and the voltage taps are also coupled to a control circuit differential amplifier and to a compensating circuit differential amplifier. While such an arrangement provides some advantages in operation of the line interface circuit, each of the multiple amplifier stages contributes a phase shift, resulting in potential instability and limited bandwidth. For a wideband line interface circuit it is necessary to minimize the number of cascaded amplifier stages which are used.
In Rosch et at. U.S. Pat. No. 4,947,427 issued Aug. 7, 1990 and entitled "Protection Arrangement For A Subscriber Line Interface Circuit" there is described an arrangement for protecting a line interface circuit from faults on the line by opening contacts of an isolation relay via which the line interface circuit is connected to the line. In this arrangement, current on the line is detected, when the relay contacts are closed, by a resistive network coupled to the feed resistors, and voltage is detected on the telephone line side of the relay contacts, so that the line voltage is detected whether or not the relay contacts are dosed. A capacitor is charged in dependence upon the detected line voltage during positive half-cycles, and in dependence upon the detected line current exceeding a threshold level during negative half-cycles, of an a.c. waveform on the line, and the capacitor voltage is compared with a threshold value for controlling the relay.
An object of this invention is to provide an improved line interface circuit.